1. Kip Irvine: Assembly Language for Intel-Based Computers Overview Stack Operations (PUSH and POP) Procedures Procedure Parameters Software Interrupts MS-DOS (INT 21h) Function Calls BIOS Keyboard Input (INT 16h) BIOS Video Control (INT 10h) Recursion

2. Kip Irvine: Assembly Language for Intel-Based Computers push 0006h push 00A5h PUSH Instruction

3. Kip Irvine: Assembly Language for Intel-Based Computers After pushing 0001 and 0002

4. Kip Irvine: Assembly Language for Intel-Based Computers Before and After Popping from the Stack pop AX ; now, AX=0002

5. Kip Irvine: Assembly Language for Intel-Based Computers Uses of the Stack Save and restore registers Save the return address when a CALL instruction is executed Push parameters on the stack before calling a subroutine Create local variables inside a procedure A procedure's stack frame includes passed parameters, the return address, and local variables.

6. Kip Irvine: Assembly Language for Intel-Based Computers main proc mov ax,@data mov ds,ax call MySub mov ax,4c00h ; returns to here int 21h main endp MySub proc. ; control transfers here. ret MySub endp Example: Calling a Procedure

7. Kip Irvine: Assembly Language for Intel-Based Computers main proc 000A call sub1 000C mov ax,.... main endp sub1 proc. call sub2 0050 ret sub1 endp sub2 proc. call sub3 0060 ret sub2 endp sub3 proc. ret sub3 endp Nested Procedure Calls (1)

8. Kip Irvine: Assembly Language for Intel-Based Computers Nested Procedure Calls (2)

9. Kip Irvine: Assembly Language for Intel-Based Computers main proc. call subroutine1. subroutine1 proc. main endp. ret subroutine1 endp Avoid Overlapping Procedures!

10. Kip Irvine: Assembly Language for Intel-Based Computers title Procedure Demonstration (SUBS.ASM) ; This program calls two procedures: one for ; keyboard input, another to add the elements ; in an array of integers..model small.stack 100h.data char db ? sum dw ? array dw 100h,200h,300h,400h,500h array_size = ($  array)/(TYPE array) ; more... Procedure Calls (1)

11. Kip Irvine: Assembly Language for Intel-Based Computers.code main proc mov ax,@data ; set up the DS register mov ds,ax call inputChar ; input char into AL mov char,AL ; store in a variable ; Prepare to call the calcSum procedure. mov bx,offset array ; BX points to array mov cx,array_size ; CX = array count call calcSum ; calculate sum mov sum,ax ; store in a variable mov ax,4C00h ; return to DOS int 21h main endp Procedure Calls (2)

12. Kip Irvine: Assembly Language for Intel-Based Computers ; input character from keyboard inputChar proc mov ah,1 ; DOS function #1: char input int 21h ; call DOS to do the work ret inputChar endp ; more... Procedure Calls (3)

13. Kip Irvine: Assembly Language for Intel-Based Computers ; Calculate the sum of an array of integers. ; Input: BX points to the array and CX contains ; the array size. Returns the SUM in AX. calcSum proc push bx ; save BX, CX push cx mov ax,0 CS1: add ax,[bx] add bx,2 ; point to next integer loop CS1 ; repeat for array size pop cx ; restore BX, CX pop bx ret ; sum stored in AX calcSum endp Procedure Calls (4)

14. Kip Irvine: Assembly Language for Intel-Based Computers Calling a NEAR Procedure main proc 0006: call sub1 0009: inc ax. main endp sub1 proc 0080: mov ax,1. ret sub1 endp

15. Kip Irvine: Assembly Language for Intel-Based Computers Calling a FAR Procedure main proc 2FC0:0006: call far ptr sub1 2FC0:0009: inc ax. main endp sub1 proc 3AB6:0080: mov ax,1. ret sub1 endp

16. Kip Irvine: Assembly Language for Intel-Based Computers Preserving Local Registers (1) Writeint proc push cx ; save registers that will change push bx push si. pop si ; restore the same registers pop bx ; (in reverse order) pop cx ret Writeint endp It is common practice to save and restore any registers that a procedure plans to modify.

17. Kip Irvine: Assembly Language for Intel-Based Computers Preserving Local Registers (2) main proc... mov cx,LIST_COUNT mov bx,DECIMAL_RADIX mov si,offset aList L1: mov ax,[si] call Writeint add si,2 Loop L1... main endp What would happen to the following program if Writeint did not preserve CX,BX, and SI?

18. Kip Irvine: Assembly Language for Intel-Based Computers Interrupts Hardware interrupts –occur as a response to a hardware device –routed through the Intel 8259 Interrupt Controller Software interrupts –calls to operating system functions, located in BIOS and resident portion of DOS –activated by the INT instruction

19. Kip Irvine: Assembly Language for Intel-Based Computers Interrupt Vectoring Process

20. Kip Irvine: Assembly Language for Intel-Based Computers INT Instruction The INT instruction is always followed by a hexadecimal number that identifies its type Common examples: –INT 10h- video BIOS –INT 14h- Serial I/O –INT 16h- keyboard BIOS –INT 17h- printer services –INT 1Ah- Time of day –INT 1Ch- User timer –INT 21h- DOS services

21. Kip Irvine: Assembly Language for Intel-Based Computers DOS Function Calls (INT 21h) The INT 21h instruction activates a DOS function call The function number (0-255) is placed in the AH register before invoking INT 21h Some functions require that you assign values to certain registers before invoking INT 21h Some functions return values in registers

22. Kip Irvine: Assembly Language for Intel-Based Computers Simple Console I/O mov ah,1 ; single character input int 21h mov ah,2 ; single character output mov dl,'A' int 21h mov ah,9 ; string output mov dx,offset message int 21h

23. Kip Irvine: Assembly Language for Intel-Based Computers 01hFiltered Input With Echo 06hDirect Input Without Waiting 07hDirect Input, No Ctrl-Break 08hDirect Input with Ctrl-Break 0AhBuffered Input 0BhGet Input Status 0ChClear Input Buffer, Invoke Input Function 3FhRead From File or Device INT 21h: Standard Input

24. Kip Irvine: Assembly Language for Intel-Based Computers Comparison of Standard Input